High pressure molecular dynamics of the partially miscible fluid mixture neon/krypton

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Abstract
The Ne/Kr fluid mixture, whose constituents have very different sizes and potentials, was studied by molecular dynamics calculations over the whole concentration range and a large temperature interval of 150–250 K, and at a pressure of about 2000 bar. Particular attention was given to the phase separation at high pressures found experimentally by Trappeniers et al. Our calculations were performed with Lennard-Jones (12–6) potentials. The calculated pressures agree well with experimental data fitted through a semi-empirical equation of state. The self-diffusion coefficients show a strong concentration dependence and exhibit no anomalities in the critical region, as it is confirmed by experiments on other systems. We observed an appreciable shift of the peaks of the radial pair-distribution functions (RDF) which cannot be explained by simply scaling the RDF of the pure substances. In the critical region the shifts of the second peak indicate a structural rearrangement in the mixture. The computation of the number of neighbours up to the first and the second coordination shell showed an accumulation of equal particles and an increase of the correlation length of the system near the critical point. The correlation length could roughly be estimated at 60 Å. This is in agreement with experiments on other binary mixtures. Furthermore, the critical concentration was found to be identical with the molar fraction where both the component molecules have equal numbers of next neighbouring particles of their own kind. It is assumed that this holds generally for partially miscible binary mixtures. We conclude that our simulation reproduces the critical region in accordance with the measurements of Trappeniers. This is confirmed by our additional calculation of the Gibbs free energy, which actually exhibits the shape necessary for a phase separation in a binary mixture.

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